The present invention relates to a device and method for repairing damaged sheet metal.
It is common to use sheet metal as a support surface for supporting articles that are being treated and/or conveyed. For example, in the wood-based panels industry, endless steel belts are used as a working sheet for conveying wood chips during the production of wood panels. The belts comprise endless belts arranged in pairs, one above the other. A mixture of wood chips, glue, and filling material is fed between two adjacent flights of the belts that are traveling horizontally at the same speed. The belts are heated and pressed, and transfer the heat and pressure to the mixture to create solidified panels of desired shape. Damage to the belts can occur if the belts are pressed against hard substances that are brought-in along with the wood chips.
It is conventional, when a belt has been damaged, to separate the belts and cut a piece out of the belt, with the damaged region being contained in the cut-out piece. For example, a piece can be cut-out of an edge of the belt if the damaged region is close to the edge. More likely, the damaged region will be near the midpoint of the belt, whereby a substantially circular piece will be cut-out. Then, a pre-manufactured replacement piece of similar size, shape and grade is welded into the hole that was formed in the damaged belt. The replacement piece is smaller than the hole, whereby a gap is formed therebetween, the gap being filled by weld material.
It will be appreciated that it is important for the edges of the replacement piece and the receiving hole to be cut with a certain amount of precision so that the replacement piece achieves a good fit therein to keep the volume of the weld seam at a minimum. That is, the gap should be of narrow and uniform width, and the mutually facing walls of the replacement piece and the hole should extend perpendicular to the plane of the sheet/replacement piece. Also, the top surfaces (and bottom surfaces) of the belt and the replacement piece and the hole should be flush (coplanar) to minimize the formation of irregularities in the surface of the repaired belt.
It is also desirable that the apparatus for cutting and/or welding be relatively light-weight, inexpensive and easy to use.
A conventional apparatus for the cutting-out of damaged areas from sheet metal and the welding-in of pre-manufactured sheet metal pieces is disclosed in Australian application No. AU-A-30143/97. That apparatus employs a guide which is immovably fixed on a metal sheet, and a carrier mounted within a circular opening of the guide for rotation relative to the guide about a center axis of the opening. The carrier has a plurality of spaced-apart supports disposed thereon for retaining respective tools, such as a piece cutting-out tool, an edge-milling tool, and a welding tool. Those tools perform their respective cutting or welding functions as the carrier is rotated by a drive motor.
The guide is immovably held on the sheet metal by either an electromagnet arrangement, or a suction mechanism. The electromagnet arrangement is heavy, and the suction mechanism uses a suction pump, which is relatively expensive. It would be desirable to employ a more economical and yet light-weight mechanism for securing the guide.
The cutting-out of the damaged area from the working sheet is performed by a plasma torch which utilizes an open flame and thus presents a fire risk. Furthermore, the plasma torch produces in the working sheet a hole having an edge characterized by poor geometrical and metallurgical properties that will inevitably require considerable finishing, e.g. milling or grinding. Also, the high heat may produce deformation and/or metallurgical changes in the areas near the cut which would require additional operations to remove them, and thus an even worse hole precision. The lower precision also leads to a wider gap between the belt and the replacement piece, requiring more filler to be inserted, and thus the need for even more heat in the welding process, thereby increasing the potential for deformations and metallurgical changes. It would thus be desirable to eliminate the need for the presence of an open flame and the need for finishing operations.
To minimize problems associated with the heat generated during welding, the conventional apparatus uses a support base attached to the guide and engaging the underside of the replacement piece. The support base carries water cooled copper elements which physically contact the weld area to conduct-away heat. However, such a water cooled arrangement adds weight and cost to the apparatus. It would be desirable to provide a cooling arrangement which is light and less costly.
After the finishing operation has been performed, the milling device must be removed from the carrier and replaced by a welding device, which is relatively time consuming. It would be desirable to minimize the time required for converting the machine between cutting and welding operations.
An additional disadvantage associated with the use of a plasma torch is that the flame cuts through the entire thickness of the metal sheet at one time, i.e., the piece being cut out is cut out in one revolution of the cutter. That means that as the torch has traveled through almost the entire cutting path, the piece being cut out will be suspended from the rest of the sheet by a continuously diminishing strip until it eventually separates. As the piece separates from the sheet, there is a clear risk that the edge of the hole will be damaged, or that a rough burr will remain. Thus, additional finishing would be needed in such a case.
One aspect of the present invention relates to an apparatus for cutting out a piece of a metal sheet. The apparatus includes upper and lower frames including upper and lower circular holes therein, respectively. Upper and lower releasable securing mechanisms are disposed on the upper and lower frames respectively, for releasably securing the upper and lower frames to upper and lower sides of the metal sheet, respectively, with the upper and lower holes disposed in alignment with one another along an axis. A center element is disposed in a first of the upper and lower frames for rotation relative to the first frame about the axis. A driving mechanism is mounted on either the center element or the first frame, for driving the center element about the axis. A chip-removing cutting tool is mounted on the center element and has a cutting edge arranged to contact the sheet for cutting a piece therefrom as the center element is rotated about the axis.
In another aspect of the invention, a center element is provided which carries a welding device arranged to weld a replacement piece in the hole as the first center element is rotated about the axis. A second center element is disposed in the other frame and is situated on a side of the metal sheet opposite the welding device. The second center element carries spaced projections for bearing against the metal sheet to form a space between the metal sheet and the second center element. A first conduit conducts inert shielding gas to an area immediately surrounding the point of welding, and a second conduit conducts inert shielding gas to the space, whereby inert shielding gas is disposed above and below the sheet.
In another aspect of the invention, a support device is carried by the same frame which carries the welding device. The support device comprises two support rollers mounted on an arm for rotation about a common axis. The support rollers are spaced apart along the axis and are arranged such that one support roller rides on the metal sheet, and the other support roller rides on the replacement piece. The support rollers are biased against the respective metal sheet and the replacement piece to keep the metal sheet flush with the replacement piece.
In still a further aspect of the invention the center element which carries the cutting tool, and the center element which carries the welding device is adapted for quick insertion and removal with respect to its frame. In that regard, either the center element or the frame carries a plurality of guide rollers. The other of the center element and the frame is engageable with the guide rollers to form therewith a radial projection/groove connection permitting the center element to rotate about its axis while preventing movement of the center element along the axis. At least one of the guide rollers is moveable radially with respect to the axis of rotation to selectively produce engagement and disengagement of the radial projection/groove connection. The center element is removable from the frame in response to such disengagement.
In still a further aspect of the invention, the frames are secured to the metal sheet by releasable securing mechanisms. Each securing mechanism includes a cup for engaging the sheet, a venturi device through which a positively pressurized gas is conducted for establishing a vacuum, and a conduit for transmitting the vacuum to the cup for adhering the cup to the sheet.
The present invention also relates to a method of welding a metal replacement piece in a hole of a metal sheet. The method comprises the steps of:
A) affixing upper and lower frames on top and bottom sides, respectively, of the metal sheet, one of the frames carrying a rotatable center element that has a welding device mounted thereon;
B) supporting the replacement piece in the hole whereby a narrow gap is formed between an outer periphery of the replacement piece and a wall of the hole;
C) rotating the center element to bring the welding device to spaced locations along the gap and making a spot weld at each of the locations using filler wire; and thereafter
D) making segment welds between respective pairs of spot welds until the entire gap has been welded.